1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a lithium-ion secondary battery.
2. Description of the Related Art
Lithium secondary batteries have high energy densities per unit weight and operating voltages, which are typically three times higher than other secondary batteries, such as nickel-cadmium (Ni—Cd) batteries and nickel-metal hydride (Ni—MH) batteries. For these reasons, research and development thereof have constantly increased.
Lithium secondary batteries can be classified into liquid electrolyte batteries and solid polymeric electrolyte batteries according to the electrolyte used. In general, batteries using a liquid electrolyte are called as lithium-ion batteries, and batteries using a polymeric electrolyte are called as lithium polymer batteries.
Lithium secondary batteries can be manufactured in various shapes. Cylindrical and rectangular shapes are frequently used for lithium-ion batteries. Since lithium polymer batteries are rather safe, light, and can be molded in various shapes, they can be more advantageously used for slim, lightweight portable electronic devices, compared to other secondary batteries.
However, when a lithium secondary battery is overcharged or short-circuited between the anode and cathode plates while assembling electrodes, an electrolyte containing lithium salts in an organic solvent decomposes at the cathode plate and the lithium metal is separated at the anode plate. This degrades battery properties and causes internal short circuits in a battery. Furthermore, heats and gases generated when the lithium secondary battery is overcharged increase the internal pressure of the battery, risking explosions or fire.
To eliminate these problems of overcharging and short circuits in the lithium secondary battery, a general canned lithium-ion battery has an internal safety device, such as a shut-down separator, a positive temperature coefficient (PTC) device for cutting off the supplied electric current when the internal temperature rises above its limit, or a safety vent for ventilation when the internal pressure rises above its limit.
The typical safety device adapted for a canned lithium-ion battery can endure an internal pressure of up to 20 kg /cm2 when overcharged. However, it has a complicated structure and is difficult to manufacture.
While canned lithium-ion batteries use a nickel-plated metal as a packaging material, plastic lithium-ion batteries are enclosed within an aluminum pouch formed by coating aluminum foil with nylon, polyethylene-co-acrylic acid (EAA), or polyethylene (PE) films. Although applications of such a metal container for secondary batteries are limited due to its rigidity, since the aluminum pouch as a packaging material for batteries is flexible, the aluminum pouch can be more easily adjusted to accommodate various sizes.
Also, the aluminum pouch is less resistant to the internal pressure increases due to the gas generation as described above than metal canning materials, so it can easily burst even with a relatively small increase in the internal pressure. Furthermore, since the gases generated within the aluminum pouch are flammable, the risk of explosion is even higher.
An example of an aluminum pouch for secondary lithium polymer batteries, capable of eliminating those problems, is disclosed in Japanese Patent Laid-open Publication No. hei 2000-100399. In the disclosure, the aluminum pouch includes thermally fusible layers capable of sealing the accommodation space of an assembly of electrode plates bound together when fused, and a region of the aluminum pouch is formed to rupture easily. The rupture-susceptible region is melted at a lower temperature than other regions and thus has a small binding strength to rupture when the internal pressure of the battery rises above a predetermined level.
However, it is difficult to form the rupture-susceptible region by selectively melting it at a lower temperature and to control the binding strength at the rupture-susceptible region according to rupture pressures. Accordingly, it is more likely to fail thereby lowering productivity.
Other examples of secondary lithium batteries are disclosed in U.S. Pat. Nos. 6,040,081 and 6,145,280.